1. Field of the Invention
The present invention relates to an imaging apparatus comprising a photoelectric conversion element, a control method thereof and an imaging system. In particular, the present invention relates to an imaging apparatus using a rolling shutter, a control method thereof and an imaging system.
2. Description of the Related Art
An imaging apparatus such as an electronic camera or video camera generally uses a technique (to be referred to as a “camera shake correction technique” hereinafter) of correcting a so-called camera shake as the apparatus becomes compact or the optical design technique such as an optical magnification improves. The camera shake correction techniques roughly fall into optical camera shake correction techniques and electronic camera shake correction techniques. An optical camera shake correction technique controls an optical path change means such as a shift lens arranged in the imaging optical system in accordance with the imaging apparatus' movement by a camera shake. An electronic camera shake correction technique displays part of a sensed image that is extracted based on the acceleration information of the imaging apparatus, thereby canceling the blur of the displayed image. The electronic camera shake correction techniques include a memory-type camera shake correction method and an extraction-type camera shake correction method. The memory-type camera shake correction method records, in a memory, an image read from an imaging element and changes the readout range for readout from the memory. The extraction-type camera shake correction method changes the readout range in reading from the imaging element.
The characteristic features of these camera shake correction techniques will be described below.
The optical camera shake correction technique is expensive because the imaging apparatus incorporates an optical path change means. Additionally, the size of the imaging optical system is large, resulting in a bulky imaging apparatus. However, since the image size does not change, the quality of a sensed image is high. To the contrary, the memory-type camera shake correction method is capable of size reduction of the imaging apparatus and feedforward control based on acceleration information from a sensed image. It is therefore easy to form an electronic camera shake correction system. However, it is necessary to always read out pixels more than display pixels and drive the imaging element at a high speed. The extraction-type camera shake correction method is also advantageous for size reduction of the imaging apparatus. Since it is only necessary to read out pixels equal in number to display pixels, the imaging element need not be driven so quickly as in the memory-type camera shake correction method. However, this method is incapable of feedforward control based on acceleration information from a sensed image. It is therefore necessary to form the electronic camera shake correction system by employing difficult feedback control or separately use an acceleration sensor such as a gyro.
As described above, each camera shake correction method has both merits and demerits. An imaging apparatus employs one of the methods in accordance with its price and marketing target. For example, an expensive high-end model sometimes forms a high-performance camera shake correction system by combining a plurality of methods.
Recently, many imaging apparatuses such as a digital camera or video camera include a CMOS image sensor in place of a CCD image sensor. A CMOS image sensor causes a floating diffusion amplifier to convert signal charges generated by a photodiode into a voltage signal. The voltage signal is read out to a column signal line for each horizontal line in accordance with a row selection signal from a vertical scanning circuit and sequentially read out to the outside in accordance with a horizontal driving signal from a horizontal scanning circuit.
Since the above-described CMOS image sensor executes voltage conversion and readout for each horizontal line, it inevitably employs a rolling shutter scheme in which the start time and end time of charge storage in pixels sequentially shift for each horizontal line. However, when control is performed to make the timing of readout of stored charges in each horizontal line match the timing of the vertical sync signal for driving the imaging element, the readout sensed image is displayed in synchronism with the vertical sync signal. The above-described extraction-type camera shake correction system using such a CMOS image sensor of a rolling shutter type has the following problems.
The extraction-type camera shake correction system using a CMOS image sensor changes the starting readout line in accordance with acceleration information. However, the readout timing of the changed starting readout line must change to, for example, the start position of the vertical sync signal. Additionally, to maintain the constant storage time of pixels, it is necessary to set the charge reset timing of the starting readout line to a predetermined timing back from the readout timing of the starting readout line. Without these changes, the display image vertical position and the storage time shift. Hence, the imaging apparatus using a CMOS image sensor does not employ the extraction-type camera shake correction system.
Japanese Patent Laid-Open No. 2000-350101 discloses a technique of adjusting the charge reset and readout timings of the starting readout line to a predetermined position in accordance with setting of the starting readout line.
A video camera drives an imaging element based on an image format such as NTSC. In, for example, NTSC, the field period is 1/16 sec=16.7 msec. For this reason, under a normal sensing condition (sensing under sufficient illumination), the storage time of the imaging element is set to 16.7 msec. The optical system, imaging element, and signal processing system are designed such that a signal read out the imaging element can obtain a sufficient imager quality as a final display image. Hence, when charge readout from the photoelectric conversion elements of a predetermined row finishes, the charges in the photoelectric conversion elements of that row are immediately reset. Then, charge storage starts in preparation for readout in the next field.
Assuming this technique, a case will be examined in which the starting readout line changes for each field, as shown in FIG. 9A. In accordance with the technique disclosed in Japanese Patent Laid-Open No. 2000-350101, the readout timing of the starting readout line is adjusted to the timing of the vertical sync signal, and the charge reset timing of the starting readout line is changed to ensure a charge storage period of 16.7 msec. In this case, it is necessary to reset charges in the next field during storage of the starting readout line in the preceding field, as shown in FIG. 9B.
As described above, the extraction-type camera shake correction system almost equally sets the unit field (or unit frame) update period and the charge storage period. For this reason, it is impossible to change the starting readout line for each unit field (or unit frame) in accordance with camera shake information.